The K-th largest number(小根堆)

【题目要求】

Given N integer numbers, find the K-th largest number. 
Requirement: design an algorithm with the time complexity O(NlogK).

【输入要求】

The first line contains N, K. In the following N lines, each line contains an integer value. 
Note that N could be very large ( 1000000 <= N <= 10000000). You should allocate the memory dynamically and design the algorithm with the time complexity O(NlogK).

12 4
7
2
1
5
6
6
4
3
8
8
10
9
【输出要求】

Output the K-th largest number.

8
【我的程序】

#include <iostream>
using namespace std;

long int num, *a;

void swapOp(long int x, long int y)
{
    long int temp=a[x]; a[x]=a[y]; a[y]=temp;
}

void downOp(long int x)
{
    while (x*2<=num)
    {
        x*=2;
        if (x+1<=num && a[x+1]<a[x]) x++;
        if (a[x]<a[x/2]) swapOp(x,x/2); else break;
    }
}

void upOp(long int x)
{
    while (x/2>=1){ if (a[x]<a[x/2]) swapOp(x,x/2); else break; x/=2; }
}

int main()
{
    long int n,k,temp;
    cin>> n>> k;  num=k;  a=new long int [k+1];

    for (long int i=1;i<=k;i++) cin>>a[i];
    for (long int i=k/2; i>=1; i--) downOp(i);

    for (long int i=k+1; i<=n; i++)
    {
        cin>> temp;
        if (temp>a[1])
        {
            swapOp(1,num); num--; downOp(1);
            a[++num]=temp; upOp(num);
        }
    }

    cout<< a[1];
    return 0;
}

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